This invention relates to a melt-extruded multicomponent monofilament having improved abrasion resistance and toughness, while maintaining its excellent dimensional stability properties. The monofilament is particularly suitable for use in the manufacture of papermaking machine forming fabrics.
In a papermaking machine, a continuous sheet of paper or paper-like material is formed by flowing a water-based slurry of cellulosic fibers onto a traveling continuous woven belt. This woven belt is known in the art as a forming fabric. As the slurry travels on the continuous belt, much of the water is removed, creating a wet paper web. Water removal is accomplished by the use of hydrofoils, table rolls, and suction boxes. A typical slurry can originally contain as little as 0.5 percent by weight of cellulosic fibers, can range in temperature from about 30xc2x0 C. to about 85xc2x0 C., and can have a pH of from 4 to 9. The wet paper web may contain 80 percent by weight water as it leaves the forming section and enters the press section.
After leaving the forming section over a couch roll, the web is transferred to a press section where a major proportion of the remaining water is removed by passing it through a series of pressure nips in sequence. On leaving the press section, the web passes to a heated dryer section for final drying. The dried web can then be calendered, to smooth the surface, and then finally collected on a reel.
Conventional forming fabrics used in the forming section of a papermaking machine are based on synthetic plastic fibers, commonly called monofilaments. For a paper machine forming fabric to be useful, it should have good abrasion resistance, toughness, and dimensional stability. It must be resistant to abrasion from contact with machine parts and from contact with solids in the cellulose fiber-water slurry. Thus, abrasion resistance refers to both the resistance against wearing of the monofilament as well as its resistance to cuts, chaffing, etc. In the same way, the monofilaments of the forming fabrics must be tough. Particularly, the monofilament must have a high degree of flex-fatigue toughness. The forming fabric must be structurally stable in the plane of the fabric, in order to cope with the stresses imposed on it during use, but also must resist stretching under the tension imposed by the powered rolls which drive the fabric in a paper making machine. It must be dimensionally stable and resist any dimensional changes in the plane of the fabric due to moisture absorption over a wide range of moisture contents, because when the machine is running it will be fully wet, and when the machine is stopped for any length of time it will dry out. Finally, the forming fabric must be resistant to degradation by various materials present in the cellulose fiber-water slurry, and in materials used to clean the forming fabric, at the prevailing temperature of use.
No known fabric exhibits perfectly all of these characteristics. The synthetic polymers which provide the currently most acceptable monofilaments used in making forming fabrics are polyesters, particularly polyethylene terephthalate, and polyamides, particularly nylon-6 (polycaprolactam) and nylon-66 (poly-hexamethyleneadipamide). These polymers have been mixed with others, such as polyethylene and polybutylene terephthalate, but still such fabrics are far from perfect.
Polyethylene terephthalate shows good chemical and dimensional stability, and also is amenable to weaving, having good crimpability, and exhibiting good heat set behavior, but its abrasion resistance leaves something to be desired, especially with higher speed modern machines.
Nylon-6 and nylon-66 show great abrasion resistance, but they have serious deficiencies for weaving because they have very poor crimpability and inadequate heat set behavior, and they possess neither adequate dimensional stability in the moisture range found in the papermaking environment, nor adequate resistance to some of the materials used in cleaning forming fabrics.
Attempts have been made in the art to combine polyester monofilaments and nylon monofilaments to obtain the advantages of each type of polymer. The inherent dimensional instability of nylon-6 and nylon-66 in the range of moisture contents found in the papermaking environment, however, running from fully wet to dry, imposes a restriction on the ratio of the number of nylon monofilaments to polyethylene terephthalate monofilaments which may be successfully used in forming fabrics.
Forming fabrics have also been prepared from monofilaments comprising blends of synthetic polymers. Two or more polymers are physically blended, and then melt-extruded to form monofilaments. Monofilaments have been described as consisting of a blend of from more than 60% to 90% by weight of polyethylene terephthalate polyester, from less than 40% by weight of a thermoplastic polyurethane, and from zero to about 5% by weight of a hydrolysis stabilizer. The disadvantage of blends is that the properties of the individual polymers are compromised when they are blended. For example, a blend containing only 60% by weight polyethylene terephthalate will not have the dimensional stability of pure polyethylene terephthalate.
A forming fabric comprising monofilaments having good abrasion resistance, toughness, and dimensional stability is desirable.
The present invention provides a melt extruded multicomponent monofilament comprising (a) an outer region comprising polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units; and (b) an inner region comprising a blend of from about 1 to about 20 percent by weight of a polyetherester block copolymer and from about 80 to about 99 percent by weight of polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units.
Using the above-described monofilament, a forming fabric for a papermaking machine can be made. Such a fabric comprises a plurality of melt-extruded multicomponent monofilaments comprising: (a) an outer region component comprising polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units; and (b) an inner region component comprising a blend of from about 1 to about 20 percent by weight of a polyetherester block copolymer and from about 80 to about 99 percent by weight of polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units.
In a preferred embodiment, the present invention further provides a forming fabric for a papermaking machine wherein the fabric comprising filaments woven in at least two directions. The at least two directions includes at least one layer of machine direction filaments and at least one layer of cross-machine direction filaments. Preferably, at least the cross-machine direction filaments comprise melt-extruded multicomponent monofilaments comprising: (a) an outer region component comprising polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units; and (b) an inner region component comprising a blend of from about 1 to about 20 percent by weight of a polyetherester block copolymer and from about 80 to about 99 percent by weight of polyethylene terephthalate or a copolyester comprising at least about 80 mole percent of ethylene terephthalate units.
Advantageously, the monofilament of the present invention has improved properties of abrasion resistance and toughness when compared to conventional monofilaments made from polyester and copolyester blends. In addition, the monofilament of the present invention exhibits dimensional stability properties at least equivalent to conventional monofilaments made from polyester and copolyester blends. These characteristics render the monofilament of the present invention particularly suitable for use in papermachine forming fabric.